PR libstdc++/56492
	* include/std/future (__future_base::_Result): Add result_type
	typedef.
	(__future_base::_S_allocate_result): Overload for std::allocator.
	(__future_base::_Task_setter): Use _Result::result_type instead of
	deducing the type from the task.
	(__future_base::_Task_state): Store allocator to allow shared state
	to be reset.  Replace std::function with member of target object type
	accessed via ...
	(__future_base::_Task_state_base): New abstract base class.
	(__future_base::_Task_state_base::_M_run): New virtual function to
	invoke type-erased target object.
	(__future_base::_Task_state_base::_M_reset): New virtual function to
	create new shared_state using same target object and allocator.
	(__future_base::__create_task_state): Allocate a new _Task_state.
	(packaged_task::packaged_task): Use __create_task_state.
	(packaged_task::reset): Use _Task_state_base::_M_reset.
	* testsuite/30_threads/packaged_task/cons/56492.cc: New.

From-SVN: r196695